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c Pleiades Publishing, Ltd., 2016. ISSN 1990-3413, Astrophysical Bulletin, 2016, Vol. 71, No. 1, pp. 14–23. c O.P. Zhelenkova, E.K. Majorova, 2016, published in Astrofizicheskii Byulleten, 2016, Vol. 71, No. 1, pp. 14–24. Original Russian Text

Search for Radio Transients and Recent Detection of Radio Sources in the RATAN-600 Surveys of 1980–1994 O. P. Zhelenkova1, 2* and E. K. Majorova1 1

Special Astrophysical Observatory, Russian Academy of Sciences, Nizhnii Arkhyz, 369167 Russia 2 ITMO University, Saint Petersburg, 197101 Russia Received April 16, 2015; in final form, July 31, 2015,

Abstract—In the paper we present the results of search for transient sources using the data from the surveys conducted on RATAN-600 at 7.6 cm in the time period of 1980–1994. We detected three events at a level of 3–5σ. A search for coincidences with detected transient events was carried out. Using the data from radio and optical surveys and the VizieR, SIMBAD, and NED databases, we made assumptions on the possible nature of these events. The first transient is probably associated with AGN activity, the second— with a cataclysmic GRB event or with a supernova, the origin of the third is not determined. The inference on the possibility of search for variable sources and transients using the data from the RATAN-600 blind surveys was drawn. Searching for transients, we have found twenty-two radio sources which are associated with the NVSS objects but are not included in the RCR catalog. Three of them turned out to be presumably variable. DOI: 10.1134/S1990341316010028 Keywords: galaxies: active—gamma-ray burst: general—supernovae: general—surveys—radio continuum: general

1. INTRODUCTION

The variability of radiation on scales of hours to tens of years is one of the characteristic properties of active galactic nuclei. The flux variation detected by telescopes occurs for many reasons. They can be variable processes in the nucleus, variations of the velocity or orientation of the relativistic jet relative to an observer, line-of-sight variations in extinction, gravitational microlensing, and flickering due to the interstellar medium. The variability of the nucleus is an important diagnostic tool in investigation of the central engine, its environment, line-of-sight properties of the matter, and, finally, evolution of the population of active galaxies.

With the appearance of wide-format optical detectors and high-performance computers, mass search for variable objects and transients became possible, and the attention to such research methods essentially increased. The same also refers to the radio range. At the end of the 20th century, beginning with the papers [1] and [2], apart from the traditional monitoring of known objects, the archive data were used in searching for variability of radio sources. Thus, the papers [3–6] included the analysis of the archive data of the NVSS [7] and FIRST [8] surveys. Let us also notice the 22-year survey of the southern sky MOST (Molonglo Observatory Synthesis Telescope) [9], the pilot survey ATCA (Australia Telescope Compact Array) at a frequency of 20 GHz (S20 GHz > 100 mJy) [10], the survey ATATS (Allen Telescope Array Twenty-Centimeter Survey), which comprises 12 epochs of observations (S1.4 GHz > 20 nJy) [11, 12], PiGSS (Pi GHz Sky Survey, Allen Telescope Array) [13, 14], and many others. Notice that archive data is frequently used in searching for long-term variability at intervals from months to a decade. *

The archive data of the surveys are also used for searching and estimating the frequency of radio transients [11–17]. They are frequently thought to be associated with different types of events and objects. They can be supernovae [18–20] or afterglows of gamma-ray bursts [21–23]. The activity of stars and compact objects in the Galaxy can be also detected as a transient event in the radio range. For the new generation of radio surveys of the sky, tidal disruption event (TDE) or tidal disruption flares (TDF) are of special interest; they are associated with a sudden increase of the accretion rate due to tidal disruption of a star, appeared too close to an object with a mass of about 106 –108 M , which can lead to

E-mail: zhe@sao.ru

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SEARCH FOR RADIO TRANSIENTS AND RECENT DETECTION OF RADIO SOURCES

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Table 1. Characteristics of the surveys conducted on RATAN-600 in 1980–1994

Survey

Δt,

Dates

H

N

days

3.94 GHz

mK

mJy

(6)

(7)

1980 51◦ 07 . 9 Mar 15, 1980–Jun 06, 1980 84 25–50 0.7

8.0

(1)

(2)

(3)

(4)

(5)

1988 51◦ 08 . 7 Dec 16, 1987–Jan 12, 1988 28

25

1.1

10.6

1993 51◦ 09 . 6 Sep 17, 1993–Nov 01, 1993 46

46

1.6

10.4

1994 51◦ 22 . 0 Apr 01, 1994–May 25, 1994 55

40

1.2

11.1

an explosion in the soft x-ray band or, probably, to radio emission [24–26]. A considerable number of radio supernovae, isolated afterglows of gamma-ray bursts, outbursts due to tidal interaction are predicted in surveys with a sensitivity of about 0.1 mJy and coverage area of about 10 sq. deg. All these events generate synchrotron radiation with self-absorption and, consequently, can be more likely observed at high radio frequencies. To search for radio sources with significant flux variability, we used the data from the radio surveys conducted with the northern sector of RATAN-600 from 1980 to 1994 [27, 28], which underwent primary processing with the RATAN-600 standard software which included rejecting defective records, deleting calibrations and noise, and stitching the scans [30]. Notice that the use of surveys in the study of the variability of radio sources with RATAN-600 offers certain advantages due to reducing the instrumental effects caused by changes in the antenna surface configuration and the feed position, which is significant in determination of flux densities for faint objects. In addition to the conducted studies using the data from the Cold surveys, namely to the compilation of the RATAN Cold Refined catalog (RCR) [31] and the search for variable radio sources in the right ascension range1 2h ≤ RA < 17h [32–34], we tried to find transient events in the averaged scans of the surveys. During the search for transient sources, twentytwo more sources were detected in the Cold survey records which were identified with the NVSS sources but had not been included in the RCR catalog. 2. TRANSIENT EVENTS As is expected, at time intervals from days to months, the sources of synchronous radiation should 1

σ, F lim

In the paper, the coordinates are given for the standard epoch 2000.0.

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dominate among transient events in the centimeter wavelength range. There is a number of events which are best interpreted based on the follow-up radio observations at other frequencies. They are of definite interest for astrophysics and moreover, could be detected in blind surveys, especially in the centimeter wave range. The known examples are the corecollapse supernovae2 [18–20], afterglows of gammaray bursts (GRB) [21–23], and also relativistic jets from tidal disruption events3 (TDE) [24, 25]. These events are basically generated by shock interaction between a fast ejection from a massive explosion and a dense gas shell. Notice that slow galactic radio transients are presumably generated by different stellar populations such as flares detected from M dwarfs, cataclysmic variables, and x-ray binaries. As a criterion of the transient nature of a source, apart from its absence in the NVSS and other catalogs, we adopted the condition of its detection in scans of only one single survey of 1980, 1988, 1993, and 1994 provided that the sensitivity of at least one another survey would be enough for its detection. The characteristics of the surveys, taken from [30, 31], are given in Table 1, where column 1 shows the name of the survey, column 2—the elevation of antenna positioning H, (3)—the begin and end dates of the survey, (4)—the duration time of the surveys Δt, in days, (5)—the number of scans N from which we obtained the averaged record, (6)—root-meansquare error of noises in the averaged scans σ, in mJy, (7)—the detection threshold Fmin in the averaged scan, in mK. Notice that the search for transient events using the data from the Cold surveys is complicated by different sensitivity of the surveys and precession, due to which the region of the survey slightly shifts. 2

For the known radio supernovae, flux densities of the order of 1–100 mJy have been detected. 3 For the well studied event Swift J164449.3+573451, the observed flux densities are about 2–20 mJy [26].

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ZHELENKOVA, MAJOROVA Table 2. Transients in the interval 7h ≤ RA < 17h in the 1980–1984 surveys Name

(1)

RA,

Dec,

F3.94 GHz ,

Ta /σ

hh mm ss.s

dd mm ss

mJy

in 1988–1994

(2)

(3)

(4)

(5)

J111417+045530 11 14 16.7 ± 0.6 +04 55 30 ± 45 21.0 ± 2.0

5.2–7.5

J113344+045030 11 33 44.1 ± 0.6 +04 50 30 ± 45 24.3 ± 2.5

3.4–4.0

J165433+045457 16 54 33.1 ± 0.3 +04 54 57 ± 45 88.2 ± 8.5

11.1–16.6

Upon the browsing the scans of the surveys in the right ascension range 2h ≤ RA < 17h , three transient events were discovered. All of them were found in the scans of the 1980 survey, which differs from the further ones by the best sensitivity, low noise, and also by longer duration (longer accumulation time). The antenna temperatures Ta of the detected events exceed 3–5σ. Transient radio sources completely meet the above requirements. Although the sensitivity of the 1988, 1993, and 1994 surveys suffices for their detection, they are not detected in the scans. The coordinate referencing of the sources was conducted using the NVSS catalog with the error in determining the right ascension of the object in the scans not exceeding a few percent of the beam pattern (BP) halfwidth. The error in declination determination for an unknown source is quite large. We can only estimate the declination from a one-dimensional scan of a sky stripe passing through the BP of the radio telescope, relying on the dependence between its halfwidth and the offset in declination from the central cross-section of the survey4 [31, 35, 36]. However, this dependence does not provide information on which side of the central cross-section the source is located, except for its passing close to the central cross-section. The source with the right ascension RA = h 11 14m 16 s.7 distinguished in the scan can have both the declination Dec = +04◦ 45 . 30 and Dec = +04◦ 55 . 30 . According to the halfwidth of the fitting Gaussian in the survey scan, its offset from the central cross-section is ±5 . If we assume the following transient coordinates: RA = 11h 14m 16 s.7 and Dec = +04◦ 45 . 30 —then it turns out that it does not satisfy the accepted transient criteria. Owing to precession, the source was observed farther from the central cross-section in 1988–1994 than in 1980. Thus, even if the event occurred, the sensitivity of the 1988–1994 surveys did not suffice for its detection. If 4

The central cross-section of the radio telescope BP passes through the central cross-section of the survey.

the transient’s coordinates were RA = 11h 14m 16 s.7, Dec = +04◦ 55 . 30 , then due to precession it was closer to the central cross-section in 1988–1994 compared to the 1980 survey, and the sensitivity of the latest surveys sufficed for its detection. The sources J113344+045030 and J165433+ 045457 pass almost through the central crosssection of the 1980 survey which excludes uncertainty in declination determination. In the 1988–1994 surveys, their antenna temperature Ta in the scans should have exceeded 5σ (see Table 2), and they should be present in the record. In Table 2, we present the names of the sources which are suspected to be transient (column 1), their right ascension (column 2), declination (column 3), and flux density (column 4) with determination errors. Column 5 shows the estimates of the ratio Ta /σ, which these sources should have had in the records of the 1988–1994 surveys with regard to the sensitivity of these surveys and the flux densities obtained from the 1980 survey. In the RATAN Cold (RC) catalog [37], which was compiled using the data from the 1980 survey, we have not found sources with the coordinates of the supposed transients given in Table 2. These sources were not included in the RCR catalog compiled using the data from the 1980–1994 surveys, as one of the conditions of including the sources into the catalog was their detection in the scans of at least two surveys. Notice that the considered data are the average from 20 to 35 of the processed records of the sky stripe passing, depending on the survey and the observing hours, which excludes the presence of random noise, moreover, the level of man-made noise near the telescope was low in 1980–1994. The scans averaged over all observations of the surveys are time intervals from one to three months. Thus, the transient events detectable in the available data will refer to slow radio transients with the duration of the event from weeks to months. Unfortunately, neither the archive of the RATAN-600 observations nor the authors of the surveys preserved daily records or groups of the averaged records that could be of use ASTROPHYSICAL BULLETIN

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Temperature

Temperature


RA

Temperature

Temperature

RA

RA

Temperature

Temperature

RA

RA

RA

Fig. 1. On the left: the regions of the averaged scan of the 1980 survey, which show the supposed transient sources J111417+045530, J113344+045030, and J165433+045457 (top to bottom). In the figures, they are marked with arrows. On the right: for comparison, with the scan of the 1994 survey where the same data regions are given.

for a more detailed analysis of the events and their interpretation. As far as the goal of the paper is to study the possibilities of detection of transient events using the observed data obtained in the RATAN600 blind survey mode but not their investigation, we considered it possible to use the preserved scans averaged over the whole set of observations to search for slow transients. Figure 1 shows the regions of averaged scans of the 1980 (on the left) and 1994 (on the right) surveys, in which the supposed transient sources J111417+045530, J113344+045030, and J165433+045457 (top to bottom) are presented. All of them are marked with arrows. ASTROPHYSICAL BULLETIN

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After the checking of the supernovae list,5 and the SIMBAD and NED databases including the catalogs of cataclysmic variables, Wolf–Rayet stars, Xray binaries, and M dwarfs, which are available in the VizieR database, we have not found any coordinate coincidence (r = 2 ) for the three transients. There is no information on X-ray sources in the considered regions in these databases. Comparing the discovered sources with the maps of the VLSSr [38], NVSS, FIRST, and GB6 [39] radio surveys we did not find 5

Central Bureau for Astronomical Telegrams: http://www.cbat.eps.harvard.edu/lists/ /RecentSupernovae.html

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ZHELENKOVA, MAJOROVA

Fig. 2. Regions of the digital sky surveys near the radio transient positions, the rectangles indicate the coordinate error boxes (3σ), the crosses are the radio transient coordinates. On the left: J111417+045530 in the SDSS image in the i filter, the box size is 27 × 135 . In the center: J113344+045030, the image from the SDSS in the i filter, the box is 27 × 135 . On the right: J165433+045457, the image from the 2MASS survey (K filter), the box is 13 . 5 × 135 .

the sources suitable to the coordinates, flux density taking into account the spectral index in the error box. We also could not find any objects coinciding in coordinates with the transients in the optical and infrared SDSS [40], LAS UKIDSS [41], and WISE survey catalogs. Based on the whole of data, we tried to interpret the detected events. J111417+045530. In the upper left panel of Fig. 1, to the left of the source J111417+045530 marked with the arrow, the source J111358+043759 was recorded, which passed quite far in declination (12 . 5) from the central cross-section of the 1980 survey, and thus it was of an extended shape. It is well seen in the 1994 survey record (upper right panel). In Fig. 2 (on the left), three faint galaxies fall into the error box: SDSS9 J111416.21+045503.7 (zph = 0.52 ± 0.15), SDSS9 J111417.26+045552.1 (zph = 0.35 ± 0.10), SDSS9 J111415.93+0455629.1 (zph = 0.65 ± 0.07), and the quasar SDSS9 J111417.06+045459.6 (zsp = 1.9281±0.0001) with several systems of absorption lines. A diffuse nebula can be seen near the quasar. Moreover, a star fall into the box. Making a search in the SIMBAD database, we did not find any peculiar galactic objects in this region. Considering the determined flux density, the radio

transient could be associated with an undetected supernova explosion in one of the three galaxies or with an afterglow of a gamma-ray burst. Let us note that the observations date from 1980, when the investigations of celestial objects in the X- and gammaray ranges had just began. One else explanation can be connected with the quasar fallen into the region, which refer to the non-active in the radio range. Its possible radio emission should be lower than 2 mJy for the NVSS survey and 11 mJy for the GB6 survey. There are studies of radio quiet quasars [42–45] at the cores of which relativistic outbursts are observed for a short periods of time [46, 47]. The position coincidence of the radio transient and the quasar is the argument for a strong relativistic ejection or possible TDE. J113344+044530. This transient (Fig. 1, the central panel) is only a little brighter in flux density than the first one (Table 2). There are no close radio sources nearby in the record. In the optical image (Fig. 2, in the center), several galaxies with the photometric redshifts z = 0.3–0.5 fall into the error box, but the positions of the transients (the cross) and the galaxy SDSS J113344.10+045026.4 (zph = 0.32 ± 0.08) almost coincide. Presumably, the radio transient is associated with an afterglow of an undetected supernova in this galaxy. ASTROPHYSICAL BULLETIN

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J165433+045457. The lower left panel of Fig. 1 shows the source J165433+045457, marked with the arrow, which is a quite narrow ejection. Apart from the narrow ejection, there is an extended structure with two “peaks.” In the scans of this sky region in other surveys, only noise is observed (as, e.g., in the lower right panel of Fig. 1). Let us remind that the record which is shown in Fig. 1 is the average of 25–50 scans (depending on the hour), which were primarily processed with the best of the 84 daily scans of the 1980 survey being selected. The presence of the extended source, which can be a superposition of three radio sources with decreasing peak flux density and increasing halfwidth, can be interpreted as a source of radio emission moving away from the central cross-section of the survey. If we take the time period between the first and third peaks and divide it by the duration of the survey, we obtain a rough estimate of angular shift, which is about 3 per day. Taking the opportunities of the HORIZONS6 system of ephemeris computation for solar-system bodies, we checked the positions of Jupiter, Saturn, Pluto, Neptune, and also the NASA Voyager1 and Pioneer11 missions but did not find any coincidence with the transient coordinates. It is possible that the source is connected with the star from the error box (GSC2.3 N3AV010380, 15 m . 4R ), which is not indicated in the catalogs as an object radiating in the radio range [48]. Thus, it is hard to tell anything definite on the origin of this transient. 3. SUPPLEMENT TO THE RCR CATALOG When identifying the RC catalog with the NVSS, we found out that about one forth of the RC sources cannot be identified with the NVSS sources [49]. The processing of the 1980–1999 surveys and the coordinate referencing to the NVSS survey allowed us to improve the coordinates and flux estimates and to get the refined RCR catalog [31]. We compared the RC and RCR sources, dividing them into groups according to right ascension hours. Five hundred forty (of 1204) sources from the RC catalog fell into the region under study including twin sources,7 and 568 sources from the RCR catalog including 16 blends and 5 binary sources the components of which are separated by the RATAN-600 beam pattern.8 6

http://ssd.jpl.nasa.gov/horizons.cgi The sources marked as “twin” [37] are ambiguous in declination determination (see Section 2). 8 Some differences with the data in quantity and composition of the radio sources presented in the paper by Soboleva et al. [31] are connected with the fact that one source, J111716.52+045041.9, was missing in the publication. After analyzing the NVSS and FIRST maps, we found that in the group of three sources marked as 7


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Table 3. Comparison of the number of sources of the RC and RCR catalogs RA, RC, RCR, RC− , RC+ , RCRn , h

obj

obj obj, (%) obj obj, (%)

(1) (2)

(3)

(4)

(5)

(6)

07

64

60

23 (36)

51

19 (32)

08

72

64

27 (38)

45

19 (30)

09

47

61

9 (19)

38

24 (39)

10

54

69

9 (17)

45

26 (38)

11

52

63

9 (17)

43

21 (33)

12

41

38

14 (34)

27

12 (32)

13

57

48

21 (37)

36

13 (27)

14

53

57

14 (26)

39

20 (35)

15

51

54

14 (27)

37

18 (33)

16

46

40

12 (26)

34

6 (15)

We calculated the percentage of non-confirmed RC sources in the RCR catalog out of the number of RC sources and compared it with the percentage of new RCR sources out of the number of those in one hour of right ascension. The derived values are given in Table 3, where columns (2) and (3) show the number of RC and RCR sources in the group after deduction of blends; (4)—non-confirmed RC sources and their percentage in the number of RC sources in an hour; (5)—confirmed RC sources; (6)—new sources and their percentage in the number of RCR sources in an hour. The average percentage of the non-confirmed RC sources over all the groups is 28%. Due to the survey band increase and accumulation of more data, about 31% RC sources new compared with the RC catalog have been detected. With the average value of the spectral index of RCR sources α3.94 GHz = 0.52, the flux at a frequency of 365 MHz attains the TXS [50] survey detection threshold of 150 mJy only for those which have S3.94 GHz ∼ 45 mJy. The number of objects fainter 45 mJy in the RCR catalog makes about 60%, i.e., quite bright sources could only be reliably identified from the TXS survey. Thus, the reference to the more sensitive NVSS survey, which appeared after the publication of the RC and TXS catalogs, allowed us to refine the declinations for the sources detected in blends, J151052.33+045206.7, J151058.7+045342.5, and J151054.66+045410.6, the last two of them turned out to be the components of a binary source.

20

ZHELENKOVA, MAJOROVA Table 4. Supplement to the RCR catalog RA Dec.

ΔRA ± err,

F ± err,

NVSS

s.ss

mJy

(1)

(2)

(3)

α3.94 GHz α0.5 GHz pr1 pr2 pr3 (4)

065848.74+045522.0 0.71 ± 0.55

25.1 ± 1.0

1.02

084022.28+050826.7 1.01 ± 0.51

72.2 ± 10.0

0.72

091924.80+051132.5 3.73 ± 1.40

65.5 ± 20.0

−0.35

091933.30+050954.1 2.20 ± 1.02

65.5 ± 20.0

−0.39

092445.92+050344.9 0.53 ± 0.05

25.5 ± 1.4

−0.27

095130.87+045101.8

(5)

(6) (7) (8) # *

−0.05

b

*

b

* #

d

*

#

d

*

# #

095133.51+045141.3 0.65 ± 0.05

12.0 ± 1.5

−0.18

100355.31+050125.1 0.23 ± 0.04

47.2 ± 4.2

0.28

*

104959.61+051953.4 2.30 ± 0.93

94.1 ± 9.5

−0.85

*

0.11

*

105010.06+043251.3 0.87 ± 0.20 100.1 ± 10.0

#

115206.57+044807.1 0.26 ± 0.05

13.0 ± 1.5

−0.12

*

120522.33+050940.9 1.60 ± 0.35

61.6 ± 11.0

−0.71

*

120944.97+044118.2 0.70 ± 0.15

31.2 ± 2.1

−0.44

*

#

120954.99+045509.7 0.30 ± 1.30

11.2 ± 2.0

−0.36

*

#

121148.67+051125.5 3.50 ± 2.07

09.8 ± 10.0

−0.21

*

133849.65+050316.0 0.91 ± 1.21

33.6 ± 11.0

0.66

?

*

#

151452.61+045635.2 0.26 ± 1.95

19.6 ± 9.0

−0.10

?

*

#

154906.14+050331.8 0.20 ± 0.90

25.1 ± 3.0

0.47

155342.56+044245.7 0.13 ± 0.04

57.1 ± 2.0

−0.69

155345.75+044225.6

#

# d

*

d

*

#

155811.18+045154.1 0.30 ± 0.05

14.5 ± 2.0

−0.20

*

#

155811.64+050538.8 0.30 ± 0.05

28.1 ± 4.0

−0.54

*

#

155851.59+050646.6 0.15 ± 0.24

59.2 ± 7.0

−0.75

160014.09+045231.0 0.40 ± 0.05

37.9 ± 6.0

0.12

160024.97+044625.5 0.94 ± 0.04 133.4 ± 13.0

the RATAN-600 deep surveys of 1980–1999 and, in doing so, to clear up the discrepancies concerning the sources from the RC catalog. Let us note that the sources detected at least in two surveys were included into the RCR catalog. A number of sources detected in one survey only were not included in the RCR catalog. The reason is that due to precession they turned out at different distance in declination from the central cross-section of the

−0.91

survey in different years.9 With increasing source’s distance from the central cross-section, the sensitivity of the survey turned out insufficient for detection. Moreover, the sensitivity of the surveys slightly changed from cycle to cycle. In an additional analysis of the survey scans, which was conducted in order to 9

The central cross-section of all the surveys coincide with the visible declination of the source SS 433.


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Fig. 3. Light curves (on the left) and spectra (on the right) of the sources with supposed variability.

discover transient signals, we singled out twenty-two sources identified with the NVSS objects. They make about 4% of the number of the sources from the RCR catalog.10 Column 1 of Table 4 shows the coordinates of these sources from the NVSS catalog; (2)—the difference ΔRA of right ascensions of the objects with the error, ΔRA = RANVSS − RA, where RANVSS are the right ascensions of the sources from the NVSS catalog, RA are the right ascensions derived from the processing of surveys’ scans. The flux densities of the sources F with the errors are given in column 3. 10

Data reduction was conducted with the first method described in [31].


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Columns 4 and 5 show spectral indices at frequencies of 3.94 GHz (α3.94 ) and 0.5 GHz (α0.5 ) (Fν ∼ ν α ). The frequency of 0.5 GHz was chosen in a similar way to [51]. Columns 6–8 present remarks, where the letter “d” indicates binary sources, “b”—objects which are not resolved in the records (so called blends). The “*” mark indicates the objects which antenna temperatures are in the range of 3σ ≤ Ta < 5σ. The antenna temperatures of other objects are 5σ. The “#” mark indicates the objects which have flux densities only in the two catalogs, NVSS and RCR (RC). Some of them have flux density estimates from the maps of the VLSSr and GB6 surveys.

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Almost half of the sources given in Table 4 have flat spectra, two of them are inverse. Four objects are quite faint with flux densities 11 mJy < F < 15 mJy, and only three objects have F > 100 mJy. Most of the sources have antenna temperatures in the survey scans within the range of 3σ ≤ Ta < 5σ. Almost half of these objects have flux density data at two frequencies only: 3.94 GHz (RCR) and 1.4 GHz (NVSS). In this list, there are objects which can be presumably variable. They are J065848+045522, J100355+050125, and J160014+045231. The source J065848+045522 was observed in the 1980, 1993, and 1994 surveys and was not detected in the records of the 1988 survey despite the sufficient sensitivity of this survey for its detection. The source J100355+050125 was observed in the 1980 survey only, although its antenna temperature in the records should have exceeded 9σ judging from comparison between its flux density and the sensitivity of the 1993 and 1994 surveys. The χ2 probability for these three objects exceeded 0.999. Figure 3 show the light curves of these sources (on the left) and their spectra (on the right). The solid circles with arrows denote the detection threshold in the surveys where the source has not been detected.

transient signals, we found that they most likely refer to three different events. One of them can be referred to transient events associated with active galactic nuclei, the second one—to afterglows in the radio band of cataclysmic GRB events or supernova explosions, and the origin of the third one is undetermined. Thus, taking into account the characteristics of the available radiometers [52] and the observation strategy, the RATAN-600 radio telescope can be used not only for monitoring of the known variable radio sources but also as a tool to search for variable and transient events.

4. CONCLUSION

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ACKNOWLEDGMENTS The paper is partially supported by grant No. 14-07-00361 from the Russian Foundation for Basic Research. The observations on the RATAN-600 radio telescope are conducted with the financial support of the Ministry of Education and Science of the Russian Federation (government contract 14.518.11.7054). The authors are grateful to the reviewer for useful comments, which helped to improve the paper. REFERENCES

Variable and transient phenomena in the radio range are associated with a wide variety of events which are typical of active galactic nuclei and galactic objects. Some events, associated with variability, are extensively studied and the results are used in investigation of the evolution and origin of activity of galaxies and peculiar stellar objects. Transient events in the radio range are more difficult to study, but highsensitive surveys with wide sky coverage are going to cardinally change the situation in the near future. In the papers we referred to here, the transients are considered most often which have been incidentally discovered in the archive data, but these discoveries allow one to estimate the frequency of event occurrence and interpret the event origin. Using the processed data from the Cold surveys conducted on RATAN-600 in 1980–1999, in a few our papers we tried to detect variable objects and transient events. The strategy of conducting the deep search Cold surveys was primarily aimed at obtaining data to search for microwave background fluctuations but not at the study of radio sources. Nevertheless, we discovered seventy-three sources suspected in variability, three transient events, and twenty-two radio sources which had not been included in the RCR catalog due to the selection criterion applied: the presence of a source in two surveys at least. In the attempt to interpret the origin of the detected


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